The present invention relates to a method of forming a multilayer plated film and, more particularly, to a multilayer plating method which is suitable when an Ni or Ni-Au plated film for installation of parts is formed on a metalized layer on a ceramic substrate.
Hitherto, as a method of forming a multilayer plated film onto a dissimilar metal, for example, as a method of electrically plating an Ni-Au film necessary for installation of parts onto a W layer, there has been known a method whereby as an underlayer to perform an electric Ni plating onto a W layer, an electroless Ni plating is first performed onto the W layer, or a method whereby in order to cover a defect of the electroless Ni plating, another electroless Ni plating is further performed thereon, or the like. This electroless Ni plated film is thermally treated in the reducing atmosphere, thereby improving the adhesive property between the Ni plated film and the underlayer metal. Due to the heat treatment, the surface of the electroless Ni plated film is polluted and decomposed or the diffusion or the like of the W layer as the underlayer metal occurs. Thus, the adhesive property between the surface of the electroless Ni paated film and a secondary Ni plated film which is formed thereon is poor.
A technique disclosed in, e.g., JP-A-No. 58-204194 or the like has been known as a conventional technique regarding such a kind of multilayer plating method.
In the multilayer plating method according to the foregoing conventional technique, no consideration is paid to a variation in the primary plated film on the dissimilar metal, a decomposition due to the heat treating process, or the like. There is a problem such that defective adhesion, discoloration or the like occurs in a secondary plated film which will be formed thereafter.